Heavy AD people have 
for mana regen.
Mages have 
and a lot more.
Tanks have no mana regen item. So I was thinking of a backwards essence reaver.
+500 hp +300 mana Passive: 2%-8% of damage taken is converted into mana.
Something like that. But mana heavy tanks need some sustain in that kind of way.
Not really needed. They generally get flat mana to last longer rather than recover. You shouldn't be spamming spells for poke and such anyway since their worth is minimal compared to someone who built damage.
There's a reason Karthus, Kassadin and Swain are more likely to build flat mana. They can burn though mana too fast for recovery to really matter. An extra 400 mana will do them more good than an extra 50% regen.
So Swain can kill you forever! Yay tank meta!
They have 
or 
It's not regen, but it's something.
Chalice of Harmony and its upgrades are also usable by tanks. They both provide MR and CDR, which are valuable tank stats. Both Catalyst upgrades provide survivability and sustain (up to level 18). Jungle tanks get mana regeneration from jungle items and blue buff.
